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Guide for determining the copyright status of a work.
Public Domain Defined
When a work is not covered by copyright (because the copyright term has expired, the creator has released the work, or the work was never copyrighted) it is in the public domain. A work that is in the public domain is a creative work that is not protected by copyright and which may be freely used by everyone.
The reasons that the work is not protected can include:
the term of copyright for the work has expired
the author failed to satisfy statutory formalities to extend the copyright
the work is a work of the U.S. Government
Works in the public domain may be used freely without the permission of the former copyright owner.